Permeability of the continental crust: dynamic variations inferred from seismicity and metamorphism

摘要

The variation of permeability with depth can be probed indirectly by various means, including hydrologic modelsnthat use geothermal data as constraints and the progress of metamorphic reactions driven by fluid flow. Geother-nmal and metamorphic data combine to indicate that mean permeability (k) of tectonically active continental crustndecreases with depth (z) according to log k u0002 )14–3.2 log z, where k is in m2nand z in km. Other independentlynderived, crustal-scale k–z relations are generally similar to this power-law curve. Yet there is also substantial evi-ndence for local-to-regional-scale, transient, permeability-generation events that entail permeabilities much highernthan these mean k–z relations would suggest. Compilation of such data yields a fit to these elevated, transientnvalues of log k u0002 )11.5–3.2 log z, suggesting a functional form similar to that of tectonically active crust, butnshifted to higher permeability at a given depth. In addition, it seems possible that, in the absence of activenprograde metamorphism, permeability in the deeper crust will decay toward values below the mean k–z curves.nSeveral lines of evidence suggest geologically rapid (years to 103nyears) decay of high-permeability transientsntoward background values. Crustal-scale k–z curves may reflect a dynamic competition between permeabilityncreation by processes such as fluid sourcing and rock failure, and permeability destruction by processes such asncompaction, hydrothermal alteration, and retrograde metamorphism.